This invention generally relates to frame synchronization, and particularly relates to synchronization of frames that are transported over an intermediate network.
Given the ever-increasing demands for voice, data, and multimedia communications, corporations are continuing to take advantage of the high-speed digital communications of the T Carrier systems. The most prominent carrier system is the T1. A T1 describes the physical layer interface to a provider network. Once the T1 carrier is in place and terminated, a customer may generate traffic. The digital signal transmitted over a T1 is referred to as digital signal level one (DS1). DS1 operates at a digital signaling rate of 1.544 Mbps.
DS1 traffic is arranged in fixed-length frames, wherein each frame consists of 193 bits of information created in 125 xcexcseconds. One bit is used for framing and the other 192 bits are used for customer traffic. The 192 bits could be data or 24 eight-bit voice channels.
For DS1 to be useful, synchronization must be maintained. Synchronization is the process that forces the receiver and transmitter to agree upon timing between the two. The receiver samples the incoming bit-stream and checks the framing bits, looking for known patterns. Sometimes synchronization is referred to as frame synchronization or alignment. Importantly, the framing bits play a key role in maintaining synchronization of frames, and any loss or corruption of a framing bit may lead to a loss of synchronization. Maintaining synchronization of the transmitted data is important to minimize data loss and corruption, as well as to maintain the circuit connection between the communicating endpoints. Typical DS1 framing and formatting is shown in FIG. 1.
In the past, DS1 traffic was carried entirely over traditional time division multiplexing (TDM) networks. There is now a movement to carry DS1 traffic over ATM (Asynchronous Transfer Mode) networks. ATM is a connection-oriented, packet-switching network technology that uses fixed-size cells to carry data. ATM requires that all cells be the same size to enable faster switching and relay across ATM switches, which make up the ATM network. Each ATM cell is 53 octets long, including a 48-octet payload preceded by a 5-octet header. Notably, other octets of the 48-octet payload may be used for the ATM adaption layer, which facilitates mapping data into and extracting data from a cell. For example, AAL1 (ATM Adaption Layer 1) has been commonly used to carry circuit switched voice information. An emerging AAL1 use is circuit emulation for carrying DS1. AAL1 uses one octet out of the 48-octet payload.
When carrying DS1 traffic over an ATM network, there is an increased risk that the ATM cells carrying the DS1 frames will periodically be lost. Any loss of cells will cause a loss of synchronization. Further, the use of ATM requires additional data manipulation and processing, such as mapping one or more DS1 frames into an ATM cell, removing the DS1 frames from the cell, and reassembling the DS1 frames for transport. This increase in data manipulation and processing increases the likelihood that the data within the frame, and in particular, the framing bits used for synchronization, will be lost or corrupted.
The synchronization techniques built into the DS1 signaling format fail to address the increased risk of signaling loss when DS1 frames are transmitted over an ATM network. Therefore, there is a need to minimize the risk of losing synchronization of data frames transmitted from one network to another over an intermediate network.
The present invention addresses the need for enhancing synchronization when frames are transmitted over an intermediate network by incorporating a synchronization code along with a data frame in a transport frame used to transport the data frame over an intermediate network. The synchronization code is generated and used to check framing and correct framing errors when the data frames are extracted from the transport frames.
The synchronization code associated with a given data frame may include the framing bits for other data frames wherein the synchronization code or codes from a first data frame is used to correct the framing bit for a second data frame. The synchronization code may also include redundant framing bits for the associated data frame. These redundant bits are used to check and correct the actual framing bit for the associated data frame.
The synchronization code may also include a sequence number within a sequence of numbers to identify lost data frames. The lost data frames may be replaced with a filler frame having a framing bit to maintain synchronization. The filler frame will not have the actual data for the data frame, but will importantly allow the recipient to receive the proper number of frames and maintain synchronization. Other aspects and features of the present invention will become apparent to those skilled in the art.